i,m a decorator and do coving all the time,best bet is to use gyproc
127mm cove,most diy stores do a plastic mitre box made by artex which is the exact size for this .also use arterx to stick it up,mix it quite thick like polyfiller
this will not set in bucket like cove adhesive,and sticks much better
butter up both sides and push into position squeezing out adhesive
then run a scraper along top and bottom to remove excess then sponge off
with a damp sponge,you shouldn,t need to nail it or caulk the gaps this way